SRAM cell structure and manufacturing method thereof

ABSTRACT

A static random access memory (SRAM) cell structure at least comprising a substrate, a transistor, an upper electrode and a capacitor dielectric layer. A device isolation structure is set up in the substrate to define an active region. The active region has an opening. The transistor is set up over the active region of the substrate. The source region of the transistor is next to the opening. The upper electrode is set up over the opening such that the opening is completely filled. The capacitor dielectric layer is set up between the upper electrode and the substrate.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the priority benefit of Taiwan application Ser.No. 92131476, filed on Nov. 11, 2003.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a memory device. More particularly, thepresent invention relates to a static random access memory (SRAM) cellstructure and manufacturing method thereof.

2. Description of the Related Art

As semiconductors having deep sub-micron features are fabricated,overall dimension of integrated circuit devices shrinks correspondingly.In the case of memory devices, overall dimension of each memory cell isalso reduced. With the development of high tech electronic products (forexample, computers, mobile phones, digital cameras or personal digitalassistants), the amount of data that needs to be stored and processincreases considerably. To meet the memory capacity demanded by theseinformation technology products, smaller and higher quality integratedmemory devices has to be developed.

Random access memory (RAM) is a type of volatile memory which has manyapplications in information technology products. In general, randomaccess memory can be classified into static random access memory (SRAM)and dynamic random access memory (DRAM).

In each SRAM cell, digital data is stored as the conductive state of atransistor. Hence, a conventional SRAM cell consists of either a set offour transistors and two resistors (4T2R configuration) or a set of sixtransistors (6T configuration). On the other hand, digital data isstored as the charging state of a capacitor inside each DRAM cell.Accordingly, a conventional DRAM cell consists of a transistor and acapacitor (including a stacked capacitor or a deep trench capacitor.

In general, SRAM has a faster data processing speed and is possible tointegrated with a complementary metal-oxide-semiconductor (CMOS)fabrication process. In other words, the process for manufacturing SRAMis simpler. However, one major drawback of incorporating SRAM is thatarea occupied by each cell is considerably larger (a SRAM cell with sixtransistors occupy an area roughly 10 to 16 times that of a DRAM cell).Yet, despite the considerable saving of area in incorporating DRAMcells, this advantage is counteracted by the complicated processes andhence the cost needed to manufacture the capacitor within each DRAMcell.

In recent years, a one transistor SRAM cell (1T-SRAM) (also referred toas a pseudo-SRAM) has been developed. The 1T-SRAM uses a DRAM memorycell (1T1C configuration) to replace the SRAM cell (6T or 4T2Rconfiguration) but is able to maintain the peripheral circuit structureof the original SRAM. Therefore, memory cell dimension is reduced andthe level of integration is increased, and yet the refresh-free propertyand small random access cycle time of a SRAM cell can be retained. Inother words, the 1T-SRAM is a potential candidate for replacing theconventional SRAM and DRAM cells.

At present, most manufacturers use the DRAM manufacturing process (forexample, in U.S. Pat. No. 6,468,855 and U.S. Pat. No. 6,573,548) tomanufacture the IT-SRAM. Since the DRAM manufacturing process involvecomplicated steps, production cost is relatively high.

SUMMARY OF THE INVENTION

Accordingly, one object of the present invention is to provide a staticrandom access memory (SRAM) cell structure and manufacturing methodthereof having a simpler processing step and yet capable of increasingthe integration level of devices and reducing the overall productioncost.

To achieve these and other advantages and in accordance with the purposeof the invention, as embodied and broadly described herein, theinvention provides a static random access memory (SRAM) cell structure.The SRAM cell has a device isolation structure to define an activeregion in a substrate. The active region has a first opening. Atransistor is set up on the active region of the substrate. Furthermore,the source region of the transistor is next to the first opening. Anupper electrode is set up over the first opening. The upper electrodecompletely fills the first opening but the upper electrode is isolatedfrom the substrate by a capacitor dielectric layer.

In the aforementioned SRAM cell structure, the device isolationstructure may further include a second opening. The second openingexposes a portion of the substrate on the sidewall of the deviceisolation structure. Furthermore, the upper electrode is set up over thesecond opening so that the second opening is completely filled.

In the aforementioned structure, because the first opening is set up inthe active region and the upper electrode is set up within the opening,the overlapping region between the upper electrode and a lower electrodeis increased. Hence, overall capacitance of the storage capacitor isincreased. In addition, if the second opening that exposes a portion ofthe substrate on the sidewall of the device isolation structure isformed, capacitance of the storage capacitor is further increased.Ultimately, the dimension of each device can be further reduced.

This invention also provides a static random access memory (SRAM) cellstructure. The SRAM cell has a device isolation structure to define anactive region in a substrate. The active region has an opening locatedwithin the device isolation structure. A lower electrode is set upwithin the opening. An upper electrode is set up over the opening suchthat the upper electrode completely fills the opening. A capacitordielectric layer isolates the upper electrode from the lower electrode.A transistor is set up on the active region of the substrate. The sourceterminal of the transistor is electrically connected to the lowerelectrode.

In the aforementioned structure, the opening is set up in the substratesuch that a portion of the opening is located within the active regionand another portion of the opening is located within the deviceisolation structure. Moreover, the upper electrode is set up within theopening so that the overlapping region between the upper electrode andthe lower electrode is increased. Hence, compared with a capacitorhaving a planar design, the capacitance of the capacitor will increase 3to 4 folds. Furthermore, with a portion of the upper electrodes set upwithin the device isolation structure, overall dimension of each devicecan be reduced.

This invention also provides a static random access memory (SRAM) cellstructure. The SRAM cell has a device isolation structure to define afirst active region and a second active region in a substrate. Thedevice isolation structure between the first active region and thesecond active region has an opening that exposes a portion of thesubstrate on the sidewall of the device isolation structure. A firsttransistor is set up over the first active region of the substrate and asecond transistor is set up over the second active region of thesubstrate. The source region of the first transistor and the secondtransistor are linked to the opening. An upper electrode is set up overthe opening. The upper electrode completely fills the opening. The upperelectrode is isolated from the substrate through a capacitor dielectriclayer.

In the aforementioned SRAM cell structures, a doped isolation region isset up in the substrate at the bottom of the device isolation structureto strengthen device isolation. Furthermore, because the opening is setup within the device isolation structure and the upper electrode is setup within the opening, overlapping region between the upper electrodeand the lower electrode is increased. Hence, compared with aconventional capacitor having a planar structure, the capacitance of thecapacitor will increase 2 to 4 folds. In addition, with the upperelectrode set up within the device isolation structure, overalldimension of each device can be reduced.

This invention also provides a method of fabricating static randomaccess memory (SRAM) cells. First, a device isolation structure isformed in a substrate to define an active region. A first opening isformed within the active region of the substrate. A dielectric layer isformed over the substrate so that a capacitor dielectric layer is alsoformed over the interior surface of the first opening. Thereafter, agate is formed over the substrate and then an upper electrode is formedover the first opening such that the upper electrode completely fillsthe first opening. Finally, source/drain regions are formed in thesubstrate using the gate and the upper electrode as a mask.

In the aforementioned method, the step of forming the first opening inthe substrate further comprises forming a second opening that exposes aportion of the substrate on the sidewall of the device isolationstructure. Furthermore, in the step of forming the dielectric layer overthe substrate and the capacitor dielectric layer on the interior surfaceof the first opening, a capacitor dielectric layer is also formed on theinterior surface of the second opening. In addition, in the step offorming the gate over the substrate and the upper electrode over thefirst opening, the upper electrode also completely fills the secondopening.

In the step of forming the first opening in the substrate, a portion ofthe first opening may lie within the device isolation structure.Furthermore, after forming the first opening in the substrate but beforeforming the dielectric layer over the substrate and the capacitordielectric layer over the first opening, a lower electrode may also beformed inside the first opening.

In the aforementioned method, the first opening is formed over theactive region and the upper electrode is formed over the first openingwith the upper electrode completely filling the first opening. Thus, theoverlapping region between the upper electrode and the lower electrodeis increased. Ultimately, the capacitance of the storage capacitor inthe SRAM cell is increased.

If the second opening is formed within the device isolation structuresuch that the capacitor dielectric layer and the upper electrode is alsoformed within the second opening, the capacitance of the storagecapacitor is further increased. Since the first opening and the secondopening are formed in the same step, no additional processes areincurred.

In addition, if a portion of the first opening is located within thedevice isolation structure, not only is the capacitance of the storagecapacitor increased but the dimension of each SRAM cell is also reducedas well.

Furthermore, the gate of the transistor and the upper electrode of thecapacitor are formed in the same step. Hence, the amount of processingand the cost of producing the SRAM cell are reduced.

This invention also provides a second method of fabricating staticrandom access memory (SRAM) cells. First, a device isolation structureis formed in a substrate so that a first active region and a secondactive region are patterned out. An opening is formed within the deviceisolation structure between the first active region and the secondactive region. The opening exposes a portion of the substrate on thesidewall of the device isolation structure. A dielectric layer is formedover the substrate so that a capacitor dielectric layer is also formedover the interior surface of the opening. Thereafter, a first gate and asecond gate are formed over the substrate and then an upper electrode isformed over the opening such that the upper electrode completely fillsthe opening. Finally, source/drain regions are formed in the substrateusing the first gate, the second gate and the upper electrode as a mask.

In the aforementioned method, after the step of forming the firstopening in the device isolation structure between the first activeregion and the second active region but before forming the dielectriclayer over the substrate and the capacitor dielectric layer over theopening, a doped isolation region may also be formed in the exposed areaat the bottom of the device isolation structure. Furthermore, in thestep of forming the opening within the device isolation structurebetween the first active region and the second active region, a portionof the opening may lie inside the first active region and the secondactive region.

In the aforementioned method, the opening is formed within the deviceisolation structure and the upper electrode is formed within the openingso that the overlapping area between the upper electrode and the lowerelectrode is increased. Hence, the capacitance of the storage capacitorcan be increased 2 to 4 times over that of a capacitor having a planarstructure. Furthermore, with the upper electrode formed inside thedevice isolation structure and the capacitor linking up both the firsttransistor and the second transistor connected to the same upperelectrode, device dimension can be further reduced.

Furthermore, the first transistor, the second transistor and the upperelectrode of the capacitor are formed in the same step. Hence, theamount of processing and the cost of producing the SRAM cell arereduced.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary, and are intended toprovide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding of the invention, and are incorporated in and constitute apart of this specification. The drawings illustrate embodiments of theinvention and, together with the description, serve to explain theprinciples of the invention.

FIGS. 1A and 1B are the respective top view and cross-sectional view ofa static random access memory cell structure according to a firstpreferred embodiment of this invention.

FIGS. 2A to 2D are schematic cross-sectional views showing the steps forfabricating the SRAM cell according to the first embodiment of thisinvention.

FIGS. 3A and 3B are the respective top view and cross-sectional view ofa static random access memory cell structure according to a secondpreferred embodiment of this invention.

FIGS. 4A to 4D are schematic cross-sectional views showing the steps forfabricating the SRAM cell according to the second embodiment of thisinvention.

FIGS. 5A and 5B are the respective top view and cross-sectional view ofa static random access memory cell structure according to a thirdpreferred embodiment of this invention.

FIGS. 6A to 6D are schematic cross-sectional views showing the steps forfabricating the SRAM cell according to the third embodiment of thisinvention.

FIGS. 7A and 7B are the respective top view and cross-sectional view ofa static random access memory cell structure according to a fourthpreferred embodiment of this invention.

FIGS. 8A to 8D are schematic cross-sectional views showing the steps forfabricating the SRAM cell according to the fourth embodiment of thisinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the present preferredembodiments of the invention, examples of which are illustrated in theaccompanying drawings. Wherever possible, the same reference numbers areused in the drawings and the description to refer to the same or likeparts.

FIGS. 1A and 1B are the respective top view and cross-sectional view ofa static random access memory cell structure according to a firstpreferred embodiment of this invention. FIG. 1B is a cross-sectionalview along line A–A′ of FIG. 1A. As shown in FIGS. 1A and 1B, the staticrandom access memory (SRAM) cell comprises a substrate 100, an accesstransistor 110 and a storage transistor 118. The substrate 100 is aP-type silicon substrate with an N-well region 108, for example.Furthermore, the substrate 100 has a device isolation structure 102 todefine an active region 104. The active region 104 has an opening 106.

The access transistor 110 is set up over the active region 104 of thesubstrate 100, for example. The access transistor 110 comprises a gate126, a gate dielectric layer 124, a source region 112 a and a drainregion 112 b. The source region 112 a of the transistor 110 is next tothe opening 106. The transistor 110 is a P-channelmetal-oxide-semiconductor (PMOS) transistor, for example.

The storage transistor 118 is set up over the opening 106 such that thetransistor 118 completely fills the opening 106. The gate 114 of thestorage transistor 118 serves as an upper electrode of a storagecapacitor. Furthermore, a portion of the gate 114 extends into theactive region 104 and the device isolation structure 102. The storagetransistor 118 is set up between the gate 114 and the substrate 100. Agate dielectric layer 116 serving as a capacitor dielectric layer forthe storage capacitor is a silicon oxide layer, a silicon oxynitridelayer or an oxide/nitride/oxide composite stack. A channel region 115 ofthe storage capacitor 118 serves as the lower electrode of thecapacitor. Furthermore, the storage transistor 1118 and the accesstransistor 1110 both use the source region 112 a.

In the aforementioned structure, the opening 106 is set up in the activeregion 104 and the storage transistor 118 is set up in the opening 106.Hence, the overlapping area between the gate 114 (the upper electrode)and the channel region 1115 (the lower electrode) is increased. Ingeneral, the capacitance of the capacitor is 2 to 4 times higher than acapacitor having a conventional planar design. Ultimately, the dimensionof each SRAM cell can be reduced.

FIGS. 2A to 2D are schematic cross-sectional views showing the steps forfabricating the SRAM cell according to the first embodiment of thisinvention. In fact, FIGS. 2A to 2D are cross-sectional views along aline A–A′ shown in FIG. 1A. In FIGS. 2A to 2D, components identical tothe ones in FIGS. 1A and 1B are labeled with identical numbers.

As shown in FIG. 2A, a substrate 100 such as a P-type silicon substrateis provided. An N-well 108 is formed in the substrate 100. Thereafter, adevice isolation structure 102 is formed in the substrate to define theactive region 104. The device isolation structure 102 is a shallowtrench isolation (STI) structure or a local oxidation of silicon (LOCOS)layer, for example. An opening 106 is formed in the substrate 100. Theopening 106 is located within the active region 104 such that theopening 106 and the device isolation structure 102 are separated fromeach other by a distance. The opening 106 is formed, for example, byperforming photolithographic and etching processes.

As shown in FIG. 2B, a dielectric layer 120 is formed over the substrate100. The dielectric layer 120 is a silicon oxide layer formed, forexample, by thermal oxidation. Obviously, the dielectric layer 120 canalso be a composite oxide/nitride/oxide layer. Thereafter, a conductivelayer 122 is formed over the dielectric layer 120. The conductive layer122 is a doped polysilicon layer formed, for example, by performing achemical vapor deposition.

As shown in FIG. 2C, the conductive layer 122 and the dielectric layer120 are patterned to form a first gate 126, a gate dielectric layer 124,a second gate 114 (the upper electrode) and a gate dielectric layer 116(the capacitor dielectric layer). The second gate 114 (the upperelectrode) completely fills the opening 106 with a portion of the gate114 covering over the device isolation structure 102. The conductivelayer 122 and the dielectric layer 120 are patterned, for example, byperforming photolithographic and etching processes.

As shown in FIG. 2D, dopants are implanted into the substrate 100 toform a lightly doped region 111 a using the first gate 126 and thesecond gate 114 (the upper electrode) as a mask. The dopants are P-typeions implanted by performing an ion implantation, for example.Thereafter, spacers 128 are formed on the sidewalls of the first gate126 and the second gate 114 (the upper electrode). The spacers 128 isformed, for example, by depositing insulating material over thesubstrate 100 to form an insulation layer (not shown) and etching backthe insulation layer anisotropically. Dopants are implanted into thesubstrate 100 using the first gate 126 and the second gate 114 (theupper electrode) as a mask to form a heavily doped region 111 b. Thedopants are P-type ions implanted by performing an ion implantation, forexample. The lightly doped region 111 a and the heavily doped region 111b together constitute the source region 112 a and the drain region 112b. The gate 126, the gate dielectric layer 124, the source region 112 aand the drain region 112 b together constitute the access transistor110. The gate 114 (the upper electrode), the gate dielectric layer 115(the capacitor dielectric layer, the source region 112 a togetherconstitute the storage capacitor 118. The channel region 115 of thestorage transistor 118 serves as a lower electrode. Hence, the gate 114(the upper electrode), the gate dielectric layer 116 (the capacitordielectric layer, the channel region 115 (the lower electrode) togetherform a capacitor. Thereafter, other processes needed to complete thefabrication of a 1T-SRAM cell are carried out. Since conventionalprocesses are used, detailed description these processes are omitted.

According to the embodiment of this invention, the opening 106 is formedwithin the active region 104 and the storage transistor is set up withinthe opening 106. Hence, the overlapping area between the gate 114 (theupper electrode) and the channel region 115 (the lower electrode) isincreased. In other words, the capacitance of the capacitor within eachSRAM cell is increased. In general, the capacitance of the capacitor isbetween 2 to 4 times more than a conventional capacitor having a planardesign so that overall dimension of each device can be reduced.Furthermore, the access transistor 110 and the storage capacitor 118serving as a capacitor are fabricated in the same step so that theamount of processing is reduced and hence the overall cost of producingthe SRAM cells is lowered.

FIGS. 3A and 3B are the respective top view and cross-sectional view ofa static random access memory cell structure according to a secondpreferred embodiment of this invention. FIG. 3B is a cross-sectionalview along line B–B′ of FIG. 3A. As shown in FIGS. 3A and 3B, the staticrandom access memory (SRAM) cell comprises a substrate 200, an accesstransistor 210 and a storage transistor 218. The substrate 200 is aP-type silicon substrate with an N-well region 208, for example.Furthermore, the substrate 200 has a device isolation structure 202 todefine an active region 204. The active region 204 has an opening 206 a.The device isolation structure 202 also has an opening 206 b thatexposes a portion of the substrate 200 on the sidewalls of the deviceisolation structure 202.

The access transistor 210 is set up over the active region 204 of thesubstrate 200, for example. The source region 212 a of the accesstransistor 210 is next to the opening 206 a. The transistor 210 is aP-channel metal-oxide-semiconductor (PMOS) transistor, for example.

The storage transistor 218 is set up over the openings 206 a and 206 bsuch that the transistor 218 completely fills the openings 206 a and 206b. The gate 214 of the storage transistor 218 serves as an upperelectrode of a storage capacitor. Furthermore, a portion of the gate 214covers a portion of the active region 204 and the device isolationstructure 202. A gate dielectric layer 216 of the storage transistor 218is set up between the gate 214 and the substrate 200, for example. Thegate dielectric layer 216 serving as a capacitor dielectric layer forthe storage capacitor is a silicon oxide layer, a silicon oxynitridelayer or an oxide/nitride/oxide composite stack. A channel region 215 ofthe storage capacitor 218 serves as the lower electrode of thecapacitor. Furthermore, the storage transistor 218 and the accesstransistor 210 both use the source region 212 a.

In the aforementioned structure, the openings 206 a and 206 b are set upwithin the active region 204 and the device isolation structurerespectively. Furthermore, the storage transistor 218 set up over theopenings 206 a and 206 b also completely fills the openings 206 a and206 b. Hence, the overlapping area between the gate 214 (the upperelectrode) and the channel region 215 (the lower electrode) isincreased. In fact, the capacitor in the second embodiment of thisinvention is able to provide even a greater capacitance than the one inthe first embodiment.

FIGS. 4A to 4D are schematic cross-sectional views showing the steps forfabricating the SRAM cell according to the second embodiment of thisinvention. In fact, FIGS. 4A to 4D are cross-sectional views along aline B–B′ shown in FIG. 3A. In FIGS. 4A to 4D, components identical tothe ones in FIGS. 3A and 3B are labeled with identical numbers.Furthermore, the steps carried out in FIGS. 4B to 4D are similar to theones in FIGS. 2B to 2D. Therefore, to simplify description, only thesteps that differ from FIGS. 2B to 2D are explained in detail.

As shown in FIG. 4A, a substrate 200 such as a P-type silicon substrateis provided. An N-well region 208 is formed in the substrate 200.Thereafter, a device isolation structure 202 is formed in the substrate200 to define an active region 204. The device isolation structure 202is either a shallow-trench isolation (STI) structure of a localoxidation of silicon (LOCOS) layer, for example. Thereafter, openings206 a and 206 b are formed in the substrate 200. The opening 206 a islocated in the active region 204 separated from the device isolationstructure 202 by a distance. On the other hand, the opening 206 b islocated in the device isolation structure 202. The opening 206 b exposesa portion of the substrate 200 on the sidewall of the device isolationstructure 202. The openings 206 a and 206 b are formed, for example, byperforming photolithographic and etching processes.

As shown in FIG. 4B, a dielectric layer 220 and a conductive layer 222are sequentially formed over the substrate 200. The conductive layer 222at least fills the openings 206 a and 206 b completely.

As shown in FIG. 4C, the conductive layer 222 and the dielectric layer220 are patterned to form a gate 224, a gate dielectric layer 226,another gate 214 (the upper electrode) and a gate dielectric layer 216(the capacitor dielectric layer). The gate 214 (the upper electrode)completely fills the openings 206 a and 206 b. Furthermore, the gate 214(the upper electrode) covers a portion of the active region 204 and aportion of the device isolation structure 202. The conductive layer 222and the dielectric layer 220 are patterned, for example, by performingphotolithographic and etching processes.

As shown in FIG. 4D, a source region 212 a and a drain region 212 b areformed in the substrate 200. Thereafter, spacers 228 are formed on thesidewalls of the gate 226 and the gate 214 (the upper electrode). Thesource region 212 a and the drain region 212 b comprise a lightly dopedregion 211 a and a heavily doped region 211 b. The gate 226, the gatedielectric layer 224, the source region 212 a and the drain region 212 btogether constitute the access transistor 210. The gate 214 (the upperelectrode), the gate dielectric layer 216 (the capacitor dielectriclayer) and the source region 212 a together constitute the storagetransistor 218. The channel region 215 of the storage transistor 218serves as a lower electrode. Hence, the gate 214 (the upper electrode),the gate dielectric layer 216 (the capacitor dielectric layer) and thechannel region 216 (the lower electrode) together constitute thecapacitor. Thereafter, other processes needed to complete thefabrication of a 1T-SRAM cell are carried out. Since conventionalprocesses are used, detailed description these processes are omitted.

According to the embodiment of this invention, the openings 206 a and 26b are formed in the active region 204 and the device isolation structurerespectively. Furthermore, the storage transistor 218 is formed over theopenings 206 a and 206 b such that the storage transistor 218 completelyfills the openings 206 a and 206 b. Hence, the overlapping area betweenthe gate 214 (the upper electrode) and the channel region 216 (the lowerelectrode) is increased. In fact, the capacitor in the second embodimentof this invention is able to provide even a greater capacitance than theone in the first embodiment.

Furthermore, the access transistor 210 and the storage transistor 218serving as a capacitor in the SRAM cell are formed in the same step.Therefore, the amount of processing and the cost of producing the SRAMcell are reduced. Moreover, both openings 206 a and 206 b are formed inthe same process. Hence, no addition processing steps are required.

FIGS. 5A and 5B are the respective top view and cross-sectional view ofa static random access memory cell structure according to a thirdpreferred embodiment of this invention. FIG. 5B is a cross-sectionalview along line C–C′ of FIG. 5A. As shown in FIGS. 5A and 5B, the staticrandom access memory (SRAM) cell comprises a substrate 300, an accesstransistor 310 and a storage transistor 318. The substrate 300 is aP-type silicon substrate with an N-well region 308, for example.Furthermore, the substrate 300 has a device isolation structure 302 todefine an active region 304. The substrate 300 has an opening 306. Aportion of the opening 306 lies within the active region 304 and aportion of the opening 306 lies within the device isolation structure302.

The access transistor 310 is set up over the active region 304 of thesubstrate 300, for example. The source region 312 a of the accesstransistor 310 is next to the opening 306. The access transistor 310 isa P-channel metal-oxide-semiconductor (PMOS) transistor, for example.

A lower electrode 315 is set up on the surface of the opening 306. Thestorage transistor 318 is set up over the opening 306 such that thestorage transistor 318 completely fills the opening 306. The gate 314 ofthe storage transistor 318 serves as the upper electrode of a storagecapacitor. Furthermore, a portion of the gate 314 extends into theactive region 304 and the device isolation structure 302. The gatedielectric layer 316 of the storage transistor 318 is set up between thegate 314 (the upper electrode) and the lower electrode 315. The gatedielectric layer 316 serving as a capacitor dielectric layer of thestorage capacitor is a silicon oxide layer, a silicon oxynitride layeror a composite oxide/nitride/oxide stack, for example. The lowerelectrode 315 is the channel region 315 of the storage transistor 318.The storage transistor 318 and the access transistor 310 both use thesame source region 312 a.

In the aforementioned structure, a portion of the opening 306 is set upin the active region 304 and a portion of the opening 306 is set up inthe device isolation structure 302. Furthermore, the storage transistor318 is set up within the opening 306. Hence, the overlapping areabetween the gate 314 (the upper electrode) and the channel region 315(the lower electrode) is increased. In general, the capacitor can have acapacitance 3 to 4 times higher than a capacitor with a conventionalplanar design. Ultimately, the dimension of each SRAM cell can bereduced.

FIGS. 6A to 6D are schematic cross-sectional views showing the steps forfabricating the SRAM cell according to the third embodiment of thisinvention. In fact, FIGS. 6A to 6D are cross-sectional views along aline C–C′ shown in FIG. 5A. In FIGS. 6A to 6D, components identical tothe ones in FIGS. 5A and 5B are labeled with identical numbers.Furthermore, the steps carried out in FIGS. 6B to 6D are similar to theones in FIGS. 2B to 2D. Therefore, to simplify description, only thesteps that differ from FIGS. 2B to 2D are explained in detail.

As shown in FIG. 6A, a substrate 300 such as a P-type silicon substrateis provided. An N-well region 308 is formed in the substrate 300.Thereafter, a device isolation structure 302 is formed in the substrate300 to define an active region 304. The device isolation structure 302is either a shallow-trench isolation (STI) structure of a localoxidation of silicon (LOCOS) layer, for example. Thereafter, an opening306 is formed in the substrate 300. A portion of the opening 306 lies inthe active region 304 and a portion of the opening 306 lies in thedevice isolation structure 302. The opening 306 is formed, for example,by performing photolithographic and etching processes.

As shown in FIG. 6B, a lower electrode 315 is formed in the opening 306.The lower electrode 315 is a silicon layer or a polysilicon layerformed, for example, by depositing silicon (or polysilicon) over thesubstrate 300 and patterning the silicon layer (or the polysiliconlayer). Thereafter, a dielectric layer 320 is formed over the substrate300. The dielectric layer 320 is a silicon oxide layer formed, forexample, by thermal oxidation. Obviously, the dielectric layer 320 canbe a composite oxide/nitride/oxide stack. A conductive layer 322 isformed over the dielectric layer 320. The conductive layer 322 is adoped polysilicon formed by chemical vapor deposition, for example.

As shown in FIG. 6C, the conductive layer 322 and the dielectric layer320 are patterned to form a gate 324, a gate dielectric layer 326,another gate 314 (an upper electrode) and a gate dielectric layer 316 (acapacitor dielectric layer). The gate 314 (the upper electrode) fillsthe opening 306 completely. The conductive layer 322 and the dielectriclayer 320 are pattered by performing photolithographic and etchingprocesses, for example.

As shown in FIG. 6D, a source region 312 a and a drain region 312 b areformed in the substrate 300. Spacers 328 are formed on the sidewalls ofthe gate 326. The source region 312 a and the drain region 312 bcomprise a lightly doped region 311 a and a heavily doped region 311 b.The gate 324, the gate dielectric layer 326, the source region 312 a andthe drain region 312 b together constitute an access transistor 310. Thegate 314 (the upper electrode), the gate dielectric layer 316 (thecapacitor dielectric layer) and the source region 312 a togetherconstitute a storage transistor 318. The lower electrode 315 serves as achannel region 315 for the storage transistor 318. The gate 314 (theupper electrode), the gate dielectric layer 316 (the capacitordielectric layer) and the channel region 315 (the lower electrode)together form a capacitor. Thereafter, other processes needed tocomplete the fabrication of a 1T-SRAM cell are carried out. Sinceconventional processes are used, detailed description these processesare omitted.

In the aforementioned structure, a portion of the opening 306 lies inthe active region 304 and a portion of the opening 306 lies in thedevice isolation structure 302. Furthermore, the storage transistor 318is set up within the opening 306. Hence, the overlapping area betweenthe gate 314 (the upper electrode) and the channel region 315 (the lowerelectrode) is increased. In general, the capacitor can have acapacitance 3 to 4 times higher than a capacitor having a conventionalplanar design. Moreover, a portion of the storage transistor 318 isformed in the device isolation structure 302 so that the dimension ofeach SRAM cell can be further reduced. In addition, the accesstransistor 310 and the storage capacitor 318 serving as a capacitor areformed in the same process. Hence, the production process is simplifiedand the cost of producing the device is reduced.

FIGS. 7A and 7B are the respective top view and cross-sectional view ofa static random access memory cell structure according to a fourthpreferred embodiment of this invention. FIG. 7B is a cross-sectionalview along line D–D′ of FIG. 7A. As shown in FIGS. 7A and 7B, the staticrandom access memory (SRAM) cell comprises a substrate 400, a firstaccess transistor 410 a, a second access transistor 410 b and a storagetransistor 418. The substrate 400 is a P-type silicon substrate with anN-well region 408, for example. Furthermore, the substrate 400 has adevice isolation structure 402 to define a first active region 404 a anda second active region 404 b. The device isolation structure 402 has anopening 406. The opening 406 exposes a portion of the substrate 400 onthe sidewall of the device isolation structure 402.

The access transistors 410 a and 410 b are set up over the active region404 a and the active region 404 b of the substrate 400, for example.Furthermore, the source region 412 a′ of the access transistor 410 a andthe source region 412 a″ of the access transistor 410 b are next to theopening 406. The access transistors 410 a and 410 b are P-channelmetal-oxide-semiconductor (PMOS) transistors, for example.

The storage transistor 418 is set up over the openings 406 such that thetransistor 418 completely fills the opening 406. The gate 414 of thestorage transistor 418 serves as an upper electrode of a storagecapacitor. Furthermore, a portion of the gate 414 extends into activeregion 404. The gate dielectric layer 416 a and the gate dielectriclayer 416 b of the storage transistor 418 are set up between the gate414 (the upper electrode) and the substrate 400. The gate dielectriclayer 416 a and the gate dielectric layer 416 b serving as capacitordielectric layer for a storage capacitor are silicon oxide layers,silicon oxynitride layers or composite oxide/nitride/oxide layers. Thechannel region 415 a and the channel region 415 b of the storagetransistor 418 serves as lower electrode of the capacitor. The storagetransistor 418 and the access transistor 410 a both use the same sourceregion 412 a′. Similarly, the storage transistor 418 and the accesstransistor 410 b both use the same source region 412 a″. An additionaldoped isolation region 407 is also formed in the substrate 400 at thebottom of the device isolation structure 402 to isolate the channelregions 415 a and 415 b.

In the aforementioned structure, the opening 406 is set up in the deviceisolation structure 402 and the storage transistor 408 is set up withinthe opening 406. Hence, the overlapping area between the gate 414 (theupper electrode) and the channel regions 415 a and 415 b (the lowerelectrode) is increased. In general, the capacitor can have acapacitance 3 to 4 times higher than a capacitor with a conventionalplanar design. Furthermore, the storage transistor 418 is set up withinthe device isolation structure 402 and hence a further reduction ofdevice dimension is permitted.

FIGS. 8A to 8D are schematic cross-sectional views showing the steps forfabricating the SRAM cell according to the fourth embodiment of thisinvention. In fact, FIGS. 8A to 8D are cross-sectional views along aline D–D′ shown in FIG. 7A. In FIGS. 8A to 8D, components identical tothe ones in FIGS. 7A and 7B are labeled with identical numbers.Furthermore, the steps carried out in FIGS. 8B to 8D are similar to theones in FIGS. 2B to 2D. Therefore, to simplify description, only thesteps that differ from FIGS. 2B to 2D are explained in detail.

As shown in FIG. 8A, a substrate 400 such as a P-type silicon substrateis provided. An N-well region 408 is formed in the substrate 400.Thereafter, a device isolation structure 402 is formed in the substrate400 to define active regions 404 a and 404 b. The device isolationstructure 402 is either a shallow-trench isolation (STI) structure of alocal oxidation of silicon (LOCOS) layer, for example. Thereafter, anopening 406 is formed in the device isolation structure 402. The opening406 exposes a portion of the substrate 400 on the sidewall of the deviceisolation structure 402. The opening 406 is formed, for example, byperforming photolithographic and etching processes. Next, dopants areimplanted into the substrate 400 at the bottom of the device isolationstructure 402 to form a doped isolation region 407.

As shown in FIG. 8B, a dielectric layer 420 is formed over the substrate400. The dielectric layer 420 is a silicon oxide layer formed, forexample, by thermal oxidation. Obviously, the dielectric layer 420 canbe a composite oxide/nitride/oxide stack. Thereafter, a conductive layer422 is formed over the dielectric layer 420. The conductive layer 422 isa doped polysilicon formed by chemical vapor deposition, for example.

As shown in FIG. 8C, the conductive layer 422 and the dielectric layer420 are patterned to form a gate 424 a, a gate 424 b, a gate dielectriclayer 426 a, a gate dielectric layer 426 b, a gate 414 (an upperelectrode), a gate dielectric layer 416 a (a capacitor dielectric layer)and a gate dielectric layer 416 b (a capacitor dielectric layer). Thegate 414 (the upper electrode) fills the opening 406 completely. Theconductive layer 422 and the dielectric layer 420 are pattered byperforming photolithographic and etching processes, for example.

As shown in FIG. 8D, a source region 412 a′ (a source region 412 a″) anda drain region 412 b′ (a drain region 412 b″) are formed in thesubstrate 400. Spacers 428 are formed on the sidewalls of the gate 426.The source region 412 a′ (the source region 412 a″) and the drain region412 b′ (the drain region 412 b″) comprise a lightly doped region 411 aand a heavily doped region 411 b. The gate 424 a, the gate dielectriclayer 426 a, the source region 412 a′ and the drain region 412 b′together constitute an access transistor 410 a. The gate 424 b, the gatedielectric layer 426 b, the source region 412 a″ and the drain region412 b″ together constitute an access transistor 410 b. The gate 414 (theupper electrode), the gate dielectric layer 416 a (the capacitordielectric layer), the gate dielectric layer 416 b (the capacitordielectric layer), the source region 412 a′ and the source region 412 a″together constitute a storage transistor 418. The channel regions 415 aand 415 b serve as the lower electrode of the storage capacitor. Thegate 414 (the upper electrode), the gate dielectric layer 416 a (thecapacitor dielectric layer), the channel region 415 a (the lowerelectrode) together constitute a capacitor adjacent to the accesstransistor 410 a. Similarly, the gate 414 (the upper electrode, the gatedielectric layer 416 b (the capacitor dielectric layer), the channelregion 415 b (the lower electrode) together constitute another capacitoradjacent to the access transistor 410 b. In other words, the capacitorserving both the access transistor 410 a and the access transistor 410 buses the same upper electrode. Thereafter, other processes needed tocomplete the fabrication of a 1T-SRAM cell are carried out. Sinceconventional processes are used, detailed description these processesare omitted.

In the aforementioned structure, the opening 406 is formed in the deviceisolation structure 402 and the storage transistor 418 is formed withinthe opening 406. Hence, the overlapping area between the gate 414 (theupper electrode) and the channel regions 415 a and 415 b (the lowerelectrode) is increased. In general, the capacitor can have acapacitance 2 to 4 times higher than a capacitor with a conventionalplanar design. Furthermore, the storage transistor 418 is formed withinthe device isolation structure 402 and the capacitor connected to theaccess transistors 410 a and 410 b uses the same upper electrode andhence a further reduction of device dimension is permitted. In addition,the access transistor 410 and the storage capacitor 418 serving as acapacitor are formed in the same process. Hence, the production processis simplified and the cost of producing the device is reduced.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the structure of the presentinvention without departing from the scope or spirit of the invention.In view of the foregoing, it is intended that the present inventioncover modifications and variations of this invention provided they fallwithin the scope of the following claims and their equivalents.

1. A static random access memory (SRAM) cell structure, comprising: asubstrate having a device isolation structure therein to define anactive region, wherein the active region has a first opening and thedevice isolation structure in the substrate has a second opening thatexposes a portion of the substrate on the sidewall of the deviceisolation structure; a transistor set up over the active region of thesubstrate, wherein a source region of the transistor is next to theopening; an upper electrode set up over and completely filled the firstopening; and a capacitor dielectric layer set up between the upperelectrode and the substrate.
 2. The SRAM cell structure of claim 1,wherein the upper electrode is also set over and completely filled thesecond opening.
 3. The SRAM cell structure of claim 1, wherein the upperelectrode covers a portion of the device isolation structure.
 4. TheSRAM cell structure of claim 1, wherein the capacitor dielectric layercomprises a composite silicon oxide/silicon nitride/silicon oxide stacklayer.